The present invention concerns a method for spectroscopically determining the concentration of alcohols having 1 to 5 carbon atoms, especially ethanol, in liquid probes such as alcohol-containing food products, healing preparations, cosmetic products and the like.
The spectroscopic determination of low alcohol concentrations, particularly ethanol, in liquid probes is widespread. With so-called near-infrared-spectroscopy (NIR), it is possible to qualitatively and/or quantitatively determine a variety of parameters. NIR-spectroscopy typically takes place in a wave length range of between about 700 nm to about 2500 nm. Superimposed or combination oscillations, rather than base oscillations, are normally measured. In this wave length range, however, the absorption capacities of the substances under investigation are relatively low, and the absorption bands are wider and frequently overlap or are superimposed. This complicates the interpretation of the measurements and can even prevent one from obtaining unambiguous and reproducible measurements. Yet, the increasing demand and opportunities for spectroscopic analysis in this wave length range and the availability of inexpensive components, such as fiberoptics, semiconductor detectors and new light sources, create a need for the fast and quick analysis of probes by means of NIR-spectroscopy with the help of simple instruments.
U.S. Pat. No. 5,679,955, for example, discloses a method of the above-named type for spectroscopically determining the concentration of low alcohols, particularly ethanol, in liquid probes. Transmission measurements in the infrared region are taken at a single wave length. This known method operates in a wave length range which is selected so that the absorption from the water contained in the probe exceeds the absorption of the additionally contained alcohol, while taking into consideration that the predominant part of such an alcohol-containing probe is formed by ethanol and water. Multiple calibration measurements are taken, each at a given wave length. The alcohol content of additional probes is determined on the basis of the previously established calibration parameters. According to this known state of the art, it is proposed to conduct the measurements at wave lengths of about 0.98 xcexcm, about 1.3 xcexcm, and about 1.45 xcexcm, to take into account the absorption coefficients for water and ethanol. A disadvantage of this prior art method is the fact that measurements must be taken at wave lengths where the absorption coefficient of water is above that for ethanol. As a result, the determination of the alcohol content has a high degree of inaccuracy, and according to U.S. Pat. No. 5,679,955 is limited to determining alcohol contents of less than 10% (volume). Thus, the known method is essentially limited to determining the alcohol concentration of beers. Beverage and general liquid probes having higher alcohol contents, such as wines, brandies, healing preparations, as well as cosmetic products, cannot be analyzed with this method. In addition, the known methods do not readily lend themselves to taking into consideration the presence of other substances, which can significantly affect the measurements.
It is an object of the present invention to provide a method for spectroscopically determining the concentration of alcohols having 1 to 5 carbon atoms, especially ethanol, in liquid probes, which permits the determination of alcohol contents, particularly ethanol concentrations, in liquid probes over a wide concentration range and with greater accuracy. This is accomplished in accordance with the present invention with a method of the above-mentioned type which is essentially characterized in that the light absorption of the probe under investigation is analyzed at at least one wave length in the range from 1100 nm to 1300 nm, and preferably in the range from 1150 nm to 1250 nm. The alcohol concentration is determined from the measured absorption with the aid of a calibration. By measuring, in accordance with the present invention, the light absorption of the probe being investigated at at least one wave length in the range from 1100 nm to 1300 nm and preferably in the range of 1150 nm to 1250 nm, the measurement takes place in a wave length range in which the absorption spectrum of the alcohol that is to be measured, particularly ethanol, has a pronounced absorption maximum. This makes it possible to readily separate and distinguish it from the absorption spectrum of water which constitutes the predominant part of such a probe. In accordance with the invention, the alcohol content of the probe under investigation is then directly determined from the measured absorption in the stated wave length range. The determination of the alcohol content makes use of a previously established calibration based on either an artificial probe, for example a water-alcohol mixture, or a real probe, for example beer, wine or the like.
In accordance with an especially preferred embodiment of the invention, it is proposed to measure the absorption in the range from 1170 nm to 1200 nm. The reason for this is that in this wave length range the absorption coefficient of ethanol has a pronounced maximum, while the absorption coefficient of water is not only essentially constant, but also lower than the absorption coefficient of ethanol. It is further preferred to take the absorption measurements in wave length ranges in which the absorption coefficient of water contained in the probe is essentially linear and preferably constant as well. This makes it possible to consider in a simple manner what part of the total measured absorption is caused by the water contained in the probe.
As already briefly mentioned above, in addition to the main constituents of the probe under investigation, low alcohol, especially ethanol, and water, other substances can at times be present which may influence the determination of the ethanol concentration in the probe being investigated. In addition to the above-discussed selection criteria of the present invention for selecting the wave length range, the present invention further proposes to preferably conduct the absorption measurement in a wave length range in which the absorption characteristics of the other substances in the probe are essentially linear. This makes it possible to take into consideration what part of the measured absorption was contributed by the other substances when determining the concentration. In this context, it is particularly preferred that the wave length is selected so that the absorption proportions contributed by the other substances essentially cancel each other at the selected wave length, or at which they can be taken into consideration in a simple manner by using corresponding, predetermined calibration constants.
To enhance the accuracy with which the alcohol concentration is determined, it is further proposed to measure the light absorption at at least two wave lengths, and preferably three wave lengths. By using at least two and preferably three wave lengths, several absorption values of the probe under investigation can be quickly determined. Simple evaluation methods make it possible to adapt and exploit the measured values and then compare them with the previously determined reference values of at least one calibration measurement. For a simple evaluation of the light absorption values of the probe under investigation at several wave lengths, the present invention proposes to preferably determine the alcohol concentration for the absorption values at the different wave lengths with a linear approximation method, for example a linear regression, a multilinear regression, or the like. Comparing the absorption values with the reference values from the calibration measurement with the help of such simple approximation methods provides adequate results without having to resort to burdensome evaluation and calculation methods.
By taking into consideration that the spectroscopic determination of alcohol concentrations at at least one wave length in a range in which the absorption coefficient of ethanol has a pronounced maximum, while the absorption coefficient of water and of other substances that may be present is essentially linear and preferably constant, the present invention makes it possible to determine the alcohol concentration in the probe under investigation over a very much larger concentration range and with corresponding accuracy. In this context, the present invention particularly prefers to use probes having ethanol concentrations of less than 60%.
To further enhance the accuracy with which the alcohol concentration is determined, it is proposed to measure the absorption at a constant probe temperature. Alternatively, or additionally, and in accordance with a further preferred embodiment of the inventive method, the temperature of the probe being analyzed is taken into consideration. Such a constant temperature or temperature stabilization of the probe makes the determination of the ethanol concentration more accurate, or the temperature dependence of particular parameters can be taken into consideration with reference to the actual probe temperature. It is further preferred that the temperature of the probe is determined with an accuracy of xc2x10.5xc2x0 C., especially xc2x10.2xc2x0 C. This helps attain the desired accuracy of xc2x10.2% by volume, and especially xc2x10.1% by volume with which the ethanol concentration is determined.